Comparative study on hygrothermal cyclic aging behavior and mechanism of adhesive-bonded similar joints

In the present investigation, four environmental cyclic aging conditions, 0–4 weeks, were applied to evaluate the durability of single-lap adhesively bonded mild steel–mild steel (MS–MS) joints. The conditions included interior humid-cyclic aging, interior thermal-humidity-freeze cycling, exterior land and air cycling, and exterior marine cycling. The adhesive joints were characterized through shear strength–displacement behavior and failure mode analysis. The results showed a progressive decline in strength under all conditions, with the most severe degradation occurring in the marine environment, where strength decreased by 69% after 4 weeks. Land and air cycling and thermal-humidity-freeze cycling produced reductions of 60% and 53%, respectively, while humid-cyclic aging resulted in a comparatively moderate decrease of 43%. Fracture surface analysis revealed that unaged joints fail cohesively within the adhesive layer, but aged joints demonstrated a shift from adhesive–cohesive mixed failure to predominantly interfacial failure. Marine-aged joints showed almost complete interfacial debonding due to chloride ion penetration and substrate corrosion. Land and air cycling promoted oxidative surface degradation, while thermal-humidity-freeze cycling induced microcracking from freeze thaw stresses. Humid-cyclic aging mainly caused adhesive plasticization and partial interfacial weakening. These findings confirm that environmental aging significantly reduces joint strength and alters failure mechanisms, with marine exposure having the greatest impact.